The present invention, in some embodiments thereof, relates to spectral imaging and, more particularly, but not exclusively, to spectral imaging using a single single-mode optical fiber.
The optical spectrum emitted from a specimen carries invaluable information on its structure, its chemical composition and physical parameters. Spectral imaging, a combination of imaging and spectroscopy, provides three dimensional data sets which contain the spectra from all the points on the imaged object. Spectral imaging has been shown useful for a wide variety of applications, including earth sciences, oceanography, homeland security, and the food industry, as well in biological and clinical applications.
Optical techniques for acquiring full spectral images often include an imaging system and spectrally dispersive element for capturing spectral information. Due to the large amount of the required information and the limited illumination intensity, a main challenge of spectral imaging is the total measurement time and the signal-to-noise ratio (SNR). In its basic form, spectral imaging is accomplished by changing colored filters in front of a camera, acquiring a series of images each with specific wavelength. Other approaches include capturing spectral cubes by spectral dispersion, using diffractive elements, such as gratings and prisms. These methods often require point-by-point scanning or line scanning, and could be combined with conventional confocal microscopes. The acquisition time in these methods depends on the scanning mechanism and the SNR which is often limited by the maximum allowable light intensity on the sample. High SNR and flexibility in selecting the spectral resolution are possible using methods which combine imaging systems with interferometers, where image acquisition is conducted by collecting interferograms of each imaged pixel and Fourier transform them into spectra.
Applications that require imaging in confined environments such as clinical endoscopy are often limited in their ability to conduct efficient spectral imaging, mainly due to limited imaging time and scanning capabilities. Spectrally encoded confocal microscopy (SECM) and spectrally encoded endoscopy (SEE), first presented in 1998 by G. J. Tearney, R. H. Webb, and B. E. Bouma, “Spectrally encoded confocal microscopy,” Opt. Lett. 23 (15), 1152-1154 (1998), which is incorporated herein by reference, use a dispersive element and a lens for angularly disperse and focus a light into a transverse line on the specimen. The reflected spectrally encoded light is measured by a spectrometer for mapping the reflectance information of the specimen. A two dimensional image is formed by slowly scanning the spectral encoded line across the sample, for example, by slowly rotating the imaging probe. Since the acquired image includes only a single wavelength emitted from each point on the sample, some modifications to the system are required in order to allow effective color imaging. A recently demonstrated color-sensitive SECM system, see Dong Kyun Kang, Dvir Yelin, Brett E. Bouma, and Guillermo J. Tearney, Optics Express, Vol. 17, Issue 17, pp. 15239-15247 (2009), which is incorporated herein by reference, required three optical fibers to simultaneously illuminate each point on the sample with three wavelengths from the red, green and blue parts of the spectrum.